1. Field of the Invention
The present invention relates to a method for calculating physical properties, and more specifically relates to a physical properties calculating method for calculating physical properties of a packaging material composed of a plurality of layers; a physical properties calculating apparatus adopting the method; and a computer program product for realizing the apparatus by a general-purpose computer system. In particular, the present invention relates to a physical properties calculating method, a physical properties calculating apparatus and a computer program product, for calculating the oxygen transmission rate as a physical property of an intermediate layer of the packaging material, which is made of, for example, ethylene-vinyl alcohol copolymer.
2. Description of Related Art
Ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH) has excellent gas barrier properties and is therefore widely used as a packaging material for packaging products to be packaged, such as foods.
The EVOH as a packaging material is formed as an intermediate layer which is sandwiched between an inner layer and an outer layer which are produced by molding resins such as polypropylene and polyethylene.
The oxygen transmission rate of the EVOH layer is an important element when packaging, especially, foods, and knowing the oxygen transmission rate of the EVOH layer is an essential condition to design the packaging material.
In the case where food is packaged by the packaging material, treatment including heating or the like for sterilization, more specifically retorting for performing sterilization at temperature around 120 degree centigrade for around several minutes to several hours, is carried out. However, when retorting is performed, the EVOH layer has such a characteristic that the oxygen transmission rate of the EVOH layer increases significantly once and then decreases gradually.
Conventionally, the oxygen transmission rate of EVOH layer is known by measurement or calculation from given conditions. For example, the “Calculation of Film's Barrier Properties” (searched on-line at URL:http://www2s.biglobe.ne.jp/˜tcfk/index. html, on Dec. 12, 2002) discloses a physical properties calculating apparatus for calculating the oxygen transmission rate of EVOH layer from the intermediate layer, inner layer, outer layer, and outside air humidity.
However, there is a problem that it is not easy to measure physical properties such as oxygen transmission rate of the EVOH layer serving as an intermediate layer.
Moreover, the physical properties calculating method disclosed in the above reference does not take into account the influence of temperature, and therefore there is a problem that this method is less practicable as a physical properties calculating method for calculating the oxygen transmission rate of EVOH layer for use in a packaging material which may be stored under low temperature conditions such as in freezers and refrigerators, or exposed to high temperatures in summer.
Furthermore, since the physical properties calculating method disclosed in the above reference does not calculate the change with time in the oxygen transmission rate during retorting, and there is a problem that this method is less practicable as a physical properties calculating method for calculating the oxygen transmission rate of EVOH layer for use in a packaging material for packaging food which requires retorting.